Many networks in organisations suffer from a lack of time synchronisation between servers, workstations and other network components. This can lead to problems processing time critical information and transactions. However, a solution has been available for many years in the form of NTP, the Network Time Protocol. NTP can be used to synchronise network time clients to an accurate time reference, or NTP server.
A stratum 1 NTP server synchronises to a precise external timing reference such as GPS or radio time & frequency transmissions and provides a precise timing resource to a local area network. There are a number of accurate external timing references available, the most well known being GPS. However, many countries also provide a radio time and frequency reference that can be used for time synchronisation.
In the UK, the MSF radio time signal is broadcast from Anthorn, Cumbria, at a frequency of 60 KHz. The German DCF-77 radio time transmissions are broadcast from Frankfurt at 77.5 KHz. The US WWVB radio time reference is transmitted from Boulder, Colorado, at 60 KHz.
There are also a number of other countries that provide similar time references. Most broadcasts operate on different frequencies and encode data differently and thus require different tuned antennas and decoding firmware. Radio time broadcasts have a finite range and are generally restricted to the national boundaries where the broadcast emanates. GPS timing references can operate anywhere in the world. However, the advantage of radio is that generally, a good signal can be obtained indoors close the host NTP time server, whereas a GPS antenna requires a good view of the sky. This can significantly reduce installation costs and simplify NTP server implementation.
A radio based NTP server generally consists of a rack-mountable time server, and a separate radio antenna. The radio antenna is used to receive the radio time and frequency broadcast. The antenna should be located in an area where a good consistent signal can be received. There are a number of factors that can affect radio time signal reception.
If the radio antenna is sited underground or in a basement, signal reception may be significantly impaired. If the antenna is located inside a metal structure or enclosure, again this may also impact signal reception. Also, if the antenna is sited too close to 'electrically noisy' equipment, such as PC monitors, can affect reception. However, usually, provided the antenna is at least 1m from a PC monitor, reception will not be affected.
Most radio time and frequency receivers consist of a ferrite, or bar, antenna contained within a plastic enclosure. The ferrite antenna generally runs the length of the antenna enclosure. Ferrite antennas are unidirectional antennas that have an ideal orientation. The ferrite antenna should be mounted in a horizontal plane at right angles to the source of the time code transmission. For example, if the source of the transmission is north of the antenna location, the antenna ferrite should be mounted horizontally pointing East-West, at right angle to North. In this orientation, the ferrite has the largest area perpendicular to the transmission.
If the antenna is rotated, as the angle to the source of transmission reduces, so the sensitivity of the antenna is reduced. Most radio time and frequency transmissions continually broadcast precise time and date information. Data is generally encoded into a series of 60 pulses, one pulse per second. Each pulse represents a data-bit that makes up the current time and date over a period of 1 minute.
When a minutes worth, or 60 pulses, have been received, the NTP server can attempt signal decoding. In this manner a time stamp should be received by the time server once each minute. If the signal drops out, or is incorrectly received, then the resulting decode will fail.
Good continuous signal reception is required to provide a reliable time reference. To summarise, a NTP server can provide an organisation with a precise timing reference for synchronising computers and network infrastructure. Time synchronisation between workstation and servers is an important aspect of data and transaction processing in most operating systems. A radio based NTP server is an ideal solution that can be easily installed and implemented to provide a solution to computer network time synchronisation issues.
This article was written by Dave Evans, a computer network design engineer that specialises in computer time and frequency systems. Dave has a number of years experience using NTP with GPS and radio time references. Click here, if you would like more information on NTP server systems.